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Internal to the torque converter is a lock-up clutch. **Usually** it is applied in the upper gears to effectively stall the converter so there is no slippage and increase fuel efficiency and reduce heat. Historically its not used in lower gears as its to jarring to the occupants when on and off the gas (feels like a manual trans). To reduce the jarring when it is engaged it must be able to drop out of LU during gear change so the T/C can slip. Also when braking it must unlock or the car will shudder. Brake pedal application drops it out of lock up (LU). My suggestion to you is to take off and get into 3rd gear and very very very lightly apply the brakes so the ECM keeps it out of LU. If doing this makes a noticeable change, (try it several times), this may be it.
One could try to find other ways to diagnose Torque converter lockup issues but often times these are terminal and lend themselves towards tranny removal.
Was there clutch material in the oil you drained? Small black particles? The test is easy. Also you may be able to lock it in 4th for example and cruise on a very slight up hill grade at 40+ mph and just tap the brake. Can you feel it nudge into LU moments after letting off the brakes? Note up hill is easier to feel than down hill or level ground.
Also a typical shift pattern would be:
1>2,2>3, 3 to lockup (3LU), 3LU>4,4>4LU,4LU>5. . . etc
Something to think about and try.